1. Field of the Invention
The instant invention relates to a metalworking apparatus including a cutting insert having cooling channels.
2. Description of the Related Art
In metalworking, a cutting insert such as the one shown in FIGS. 5 and 6 is often used to machine metals, such as by forming threads, on a lathe. The cutting insert 90 and a chip breaker 100 are held in a fixture 200. As shown in FIG. 6, the cutting insert has a cutting edge 92 that cuts the metal workpiece 1, forming a chip 3 consisting of the material removed from the workpiece. The chip breaker 100 breaks the chip so that the chip does not become too long, difficult to handle and dispose of.
Metalworking involves heat creation so, as shown in FIG. 6, cooling channels 80 are often provided to supply a cooling liquid to the workpiece where the cutting takes place. In the device shown in FIG. 6, the cooling channels are disposed in the chip breaker 100. However, cooling channels may be disposed in the cutting insert, as disclosed in U.S. Pat. No. 6,447,218 and German Publication No. 3740814.
More recently, it has been desirable to enable the use of the highest possible pressure in the cooling liquid and to supply the liquid in the form of one or more jets mainly directed towards the cutting insert and the chip, because as the pressure used in the liquid jet increases, the ability of the liquid jet to break up the chip increases. Liquid pressures as high as 2,800 bar are known, as disclosed in U.S. Pat. No. 5,148,728.
Notwithstanding the chip breaking effect of high pressure liquid, when a cutting insert, during an operation such as turning, cuts loose a chip from a rotating workpiece, usually of metal, considerable amounts of heat are generated. The actual cutting of the chip takes place in a primary shear zone, which is developed in a peripheral portion of the workpiece and extends obliquely upwards from the cutting edge of the cutting insert. By virtue of the high temperatures in the chip, the workpiece and cutting insert, the chip separated in the primary shear zone cannot slide away across the top side of the cutting insert without being influenced by both friction and adherence.
The very hot chip adheres to the top surface of the cutting insert along a certain contact length. The contact length extends away from the shear zone, which is near the cutting edge, a distance ranging from tenths of a millimeter to a few millimeters along the top of the cutting insert, depending on the material of the workpiece.
To remove the chip from the surface of the cutting insert and to break up the chip, modern high-pressure, cooling-liquid technology aims at introducing the cooling-liquid jet into the substantially wedge-shaped space provided between the bottom side of the chip and the top side of the cutting insert at the point where the chip is initially separated from the cutting insert. The idea is to form a so-called hydraulic wedge between the chip and the top side of the cutting insert so that the wedge can contribute to “break out” the chip and, as far as possible, reduce the extent of the contact length of the chip along the cutting insert. However, the attempts to improve the cooling and the flow of the chip away from conventional cutting insert carried out hitherto have not been entirely successful because of the coatings used on cutting inserts and the placement of the cooling channels.
In general, a threading insert has a tungsten carbide (WC) body or the like, and the surface has a special, very hard, ceramic coating for extending tool life, for example Titanium Nitride (NTi). We have recognized, however, that the hardening coatings are poor conductors. Moreover, the cooling channels are sometimes obstructed by the chip flow and therefore heat removal decreases. In addition, in conventional threading inserts, the cooling channels are coated with tungsten carbide, which reduces the effectiveness of the cooling liquid.
There is a need in the art for a cutting insert that is effectively cooled, yet which has a hardening coating.